I believe that libraries should be for free choice reading because it helps kids to learn to love books and reading. I also believe it is ok if a kid chooses books that are too hard or too easy. Books that are too hard are perfect for sharing with a parent. If a child is really motivated to learn about something, they may push themselves to read through a too-hard book and may pick up more than one would anticipate from a difficult text. Easy books tend to build confidence in reading abilities. If children are confident in their reading abilities, they are less likely to shy away or give up on more difficult on-level text presented to them.
Nurturing a love of reading through free choice is the first step in becoming a good reader. However, kids also need to be reading on-level books to advance their reading skills. The following is a guideline that teachers sometimes use to help kids choose "just right" books.
THE FIVE FINGER RULE
Open a book to any page. Hold up a finger each time you see a word you don't know.*
0-1 Finger - This book might too easy.
2-3 Fingers - This book is probably just right.
4 Fingers - This book might be a bit of challenge.
5+ Fingers - This book is too hard and you should consider looking for something else if you want to read independently.
*Names and proper nouns do not need to count in regard to number of unknown words.